A promising approach to analysis of the large rapid sensitivity change occurring in very early dark adaptation appears to lie in the yet unexplored case of dark adaptation following extremely intense preadapting lights. The very early dark adaptation curves of normal persons will be studied following very intense bleaching lights, to yield data for the cones; and similar cures from rod monochromats will be studied to obtain early high intensity rod curves unobscured by cone activity. In both cases it is predicted from a consideration of late receptor potentials that the early drop will be much reduced compared to present data. A new apparatus has been designed for the necessary high-intensity measurements. Procedures will be similar to those used earlier by the principal investigator in other studies of very early dark adaptation. The outcome is expected to support or reject the theory that the fluctuations of visual sensitivity during very early dark adaptation are associated with the change in the late receptor potential that follows cessation of an adapting light.